When moderately strong earthquake happens, things, which are not strongly attached to the floor, fall around. Inside of cave, pieces of rocks would fall from the ceiling. Also there would be dust in the air and cracks everywhere in case of more strong earthquake.
When I tried to depict earthquake in my comics, I did it like this:
Not sure, if it's the best way to do it (LOL), but maybe you'll use something from it for your work, too.